1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to apparatus for the generation of combined compressional and shear waves.
2. Description of the Prior Art
Injection of seismic signals into the earth by means of apparatus which utilizes compressed gas to propel an impact mass against a rigid earth contacting target as described in detailed in U.S. Pat. Nos. 4,284,164 and 4,284,165, both issued Aug. 18, 1981 and entitled "Acoustic Pulse Generator". A significant advantage of such devices is their ability to produce rapidly repeated seismic signals of variable carefully controlled energy content. Devices of these character are therefore well suited to the production of combined compressional and shear waves.
Recent advances in the ability of seismic data acquisition and processing equipment to handle large amounts of data have stimulated greater interest in the development of versatile high seismic sources adapted to produce both compressional and shear waves. By recording the full waveform of a seismic wave, including P, SH and SV waves, one can get far more detailed information concerning rock properties and depth. With the aid of the invention to be described, devices of the type disclosed in the above referenced patents constitute efficient means for producing such combined compressional and shear waves.
The actuator assembly for seismic sources such as described in the above referenced patents typically includes an impact mass weighing several thousands of pounds which must be propelled downwardly by compressed gas or the like against a rigid target resting on the earth beneath it. In order to generate a desired combination of P, SH and SV waves, it is necessary to be able to operate the source so that this heavy impact mass will travel along a selected slant path to the target. The term "full waveform seismic sourcep" as used in this application means a device which can be manipulated in position so that this slant path may be readily varied through 360 degress of aximuth at 90 degree intervals. Such a slant path needs to be achieved with precision, that is to say, within a degree or so of a prescribed inclination and azimuth. Furthermore, the mechanism whereby this can be accomplished, practically speaking, must be transportable by means of a highly mobil vehicle from place to place along an extended line of survey. Achieving the kind of accuracy and maneuverability which is required for these purposes in a vehicle mounted system presents a challenge.
It is therefore a general object of this invention to provide a seismic source with improved means for mounting and controlling such source which delivers energy to a selected point on the earth along any of a plurality of selected slant paths.
It is a more particular object of this invention to provide such a seismic source wherein a heavy impact mass is accelerated downwardly along any such slant paths to impact a rigid earth-contacting target.
Other and further objects and advantages of the invention will becme apparent from a consideration of the detailed description and drawings to follow taken in conjunction with the appended claims.